Untitled

<!DOCTYPE html>
<html>

<head>
    <meta charset="UTF-8">
    <title>title</title>

    <script>

            var callbackWhenDataGetsBack = function(data) {
                console.log(JSON.parse(data));
            }

            function loadUsers(url, callback) {
                    console.log("preparing request");

                var xhr = new XMLHttpRequest();

                xhr.onreadystatechange = function () {
                    if (xhr.readyState === 4) {
                        console.log("data received");
                        callback(xhr.response);
                    }
                }

                xhr.open('GET', url, true);
                xhr.send('');
                console.log("sent");
            }

        window.onload = function () {
            loadUsers("https://kartoteka-project.firebaseio.com/users.json", callbackWhenDataGetsBack);
        };
    </script>
</head>

<body>
    <script src="https://code.jquery.com/jquery-3.4.1.min.js" integrity="sha256-CSXorXvZcTkaix6Yvo6HppcZGetbYMGWSFlBw8HfCJo="
        crossorigin="anonymous"></script>
</body>

</html>